1. Field of the Invention
This invention relates to a converging lens for converging parallel beam of light or a collimator lens for collimating, divergent beam of light. In particular, this invention relates to a lens installed in an optical system in which a semiconductor laser is used as a light source.
2. Description of the Prior Art
An optical system of an optical information recording/reproducing apparatus, such as optical disk apparatus etc. includes a light source having a semiconductor laser, and an objective lens converging (focusing) the luminous flux emitted from the light source onto an optical disk, and a signal detecting optical system which is adapted to read information recorded in the disk and error signals by reflected light from the optical disk.
Also, the objective lens is mounted on an actuator which is designed such that the objective lens can be finely moved at least in the optical axis direction thereof (focusing servo) so that an out of focus caused by warping of the disk, etc. can be corrected.
In a read only type apparatus, a plastic lens of which surfaces are aspherical is used as an objective lens so as to take the objective lens lightly. On the other hand, a glass molded lens is used as a collimator lens to avoid focal length change by change of temperature and/or humidity.
In any event, since it cannot correct a chromatic aberration with one lens,in a system in which record and reproduction are possible, the chromatic aberration correction lens is used. This type lens is constituted from an element of 4 pieces, in general.
However, in the 4 piece construction, there are problems that the arrangement space becomes large and the cost becomes high.